Heat pipe with composite capillary wick structure

ABSTRACT

A heat pipe includes a metal casing ( 100 ) having an evaporating section ( 400 ) and a condensing section ( 600 ). A composite wick structure is provided in the evaporating section. The composite wick structure comprises at least two wick layers in a radial direction of the heat pipe, and one of the at least two wick layers is a plurality of fine grooves ( 200 ) defined in an inner surface of the evaporating section. A singular wick structure is provided in the condensing section. An average capillary pore size of the evaporating section is smaller than that of the condensing section.

FIELD OF THE INVENTION

The present invention relates generally to a heat transfer apparatus, and more particularly to a heat pipe having composite capillary wick structure.

DESCRIPTION OF RELATED ART

As a heat transfer apparatus, heat pipes can transfer heat rapidly and therefore are widely used in various fields for heat dissipation purposes. For example, in electronics heat pipes are commonly applied to transfer heat from heat-generating electronic components, such as central processing units (CPUs), to heat dissipating devices, such as heat sinks, to thereby remove the heat away. A conventional heat pipe generally includes a sealed casing made of thermally conductive material and a working fluid contained in the casing. The working fluid is employed to carry heat from one end of the casing, typically called an “evaporating section”, to the other end of the casing, typically called a “condensing section”. Specifically, when the evaporating section of a heat pipe is thermally attached to a heat-generating electronic component, the working fluid receives heat from the electronic component and evaporates. Then, the generated vapor moves towards the condensing section of the heat pipe under the vapor pressure gradient between the two sections. In the condensing section, the vapor is condensed to liquid by releasing its latent heat to, for example, a heat sink attached to the condensing section. Thus, the heat is removed away from the electronic component.

In order to rapidly return the condensed liquid back from the condensing section to the evaporating section to start another cycle of evaporation and condensation, a capillary wick is generally provided in an inner surface of the casing in order to achieve the return of the liquid. In particular, the liquid is drawn back to the evaporating section by a capillary force developed by the capillary wick. The capillary wick may be a plurality of fine grooves defined in its lengthwise direction of the casing, a fine-mesh wick, or a layer of sintered metal or ceramic powders. However, the capillary force derived from each type of these wicks is generally different, and meanwhile, the flow resistance provided by each type of wick may also be different. The general rule is that as average capillary pore size grows larger, capillary force grows smaller and provides a lower flow resistance.

FIG. 4 shows an example of a related heat pipe. The heat pipe includes a metal casing 10 and a singular uniform capillary wick 20 attached to an inner surface of the casing 10. The casing 10 includes an evaporating section 40 at one end and a condensing section 60 at the other end. An adiabatic section 50 is provided between the evaporating and condensing sections 40, 60. The adiabatic section 50 is typically used for transport of generated vapor from the evaporating section 40 to the condensing section 60. The wick 20 is uniformly arranged against the inner surface of the casing 10 from its evaporating section 40 to its condensing section 60. However, this singular and uniform-type wick 20 generally cannot provide optimal heat transfer effect for the heat pipe because it cannot obtain simultaneously a large capillary force and a low flow resistance.

In view of the above-mentioned disadvantage of the conventional heat pipe, there is a need for a heat pipe having a good heat transfer.

SUMMARY OF THE INVENTION

The present invention relates to a heat pipe. In one embodiment, the heat pipe includes a metal casing having an evaporating section and a condensing section. A composite wick structure is provided in the evaporating section. The composite wick structure comprises at least two wick layers in a radial direction of the heat pipe, one of the at least two wick layers is a plurality of fine grooves defined in an inner surface of the evaporating section. A singular wick structure is provided in the condensing section. An average capillary pore size of the evaporating section is smaller than that of the condensing section. The singular wick provides a low flow resistance in the condensing section so that the liquid condensed in the condensing section can flow more easily through the condensing section to reach the evaporating section. Meanwhile, the composite wick structure develops a large capillary force to draw the liquid from the condensing section to the evaporating section. Thus, the condensed liquid is brought back from the condensing section to the evaporating section in an accelerated manner, thereby increasing the total heat transfer capacity of the heat pipe.

Other advantages and novel features of the present invention will become more apparent from the following detailed description of the embodiment when taken in conjunction with the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present device can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present device. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.

FIG. 1 is a longitudinal sectional view of a heat pipe in accordance with first embodiment of the present invention;

FIG. 2 is a cross-sectional view of an evaporating section of the heat pipe of FIG. 1;

FIG. 3 is similar to FIG. 2 by showing a second embodiment of the evaporating section; and

FIG. 4 is a longitudinal sectional view of a related heat pipe.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 show a heat pipe in accordance with one embodiment of the present invention. The heat pipe includes a metal casing 100 made of highly thermally conductive materials such as copper or copper alloys, a working fluid (not shown) contained in the casing 100 and a composite capillary wick (not labeled) arranged in an inner surface of the casing 100. In this embodiment, the casing 100 includes an evaporating section 400 at one end, a condensing section 600 at the other end and an adiabatic section 500 arranged between the evaporating section 400 and the condensing section 600.

The working fluid functions as a heat carrier for transferring heat from the evaporating section 400 to the condensing section 600. In particular, the working fluid contained in the evaporating section 400 absorbs heat from a heat source and evaporates, and then carries the heat to the condensing section 600 in the form of vapor. Then, the vapor releases its heat to ambient environment and is condensed back to a liquid state. The condensed liquid is then brought back to the evaporating section 400 via the composite capillary wick.

The composite capillary wick includes a plurality of fine grooves 200 (and is therefore hereafter referred to as “groove-type wick”) defined in the evaporating, adiabatic and condensing sections 400, 500, 600, a layer of porous sintered powders 210 (hereafter referred to as a “sintered-type wick”) formed in the evaporating section 400 by a sintering process and a layer of fine mesh 220 (hereafter referred to as “mesh-type wick”) defined in the evaporating section 400. The grooves 241 extend in the lengthwise direction of the casing 100 and may be formed by mechanical machining. The grooves 200 can also be defined not in the adiabatic and condensing sections but in another type of wick directly arranged at the adiabatic and condensing sections 500, 600. Along the longitudinal direction of the heat pipe, the average pore sizes of the evaporating, adiabatic and condensing sections 400, 500, 600 gradually increase. In the evaporating section 400, the grooves 200 are filled with the sintered powders 210, and the mesh-type wick 220 is attached to the sintered-type wick 210 such that along a radial direction of the evaporating section 400, from the groove-type wick 200 to the mesh-type wick 220 which means from the outer layer to the inner layer of the composite capillary wick, the pore sizes of the composite capillary wick are gradually increased.

In this embodiment, the pore size of the groove-type wick 200 at the evaporating section 400 is much smaller than that at the adiabatic and condensing sections 500, 600, since the grooves 200 at the evaporating section 400 are filled with the sintered powders 210. The groove-type wick 200 at the adiabatic and condensing sections 500, 600 has a relatively large average capillary pore size and therefore provides a relatively low flow resistance to the condensed liquid to flow therethrough to the evaporating section 400. The sintered powders 210 filled in the grooves 200 of the evaporating section 400 have a relatively small average capillary pore size and accordingly develops a relatively large capillary force. As a result, the groove-type wick 200 reduces the flow resistance the condensed liquid encounters when flowing through the condensing and adiabatic sections 600, 500, and, as the sintered-type wick 210 has a large capillary force it can therefore rapidly drawn the condensed liquid back to the evaporating section 400 from the adiabatic and condensing sections 500, 600. And meanwhile, the mesh-type wick 220 is attached on the sintered-type wick 210, which accelerates evaporation of the vapor to separate the vapor into more micro-vapor. Thus, as a whole, the cycling of the working fluid is accelerated and therefore the total heat transfer capacity of the heat pipe is enhanced.

FIG. 3 illustrates an evaporating section of a heat pipe according to a second embodiment of the present invention. In this embodiment, from the outer layer to the inner layer of the composite capillary wick, the pore sizes of the composite capillary wick are gradually reduced, the composite capillary wick of the evaporating section in series comprises a groove-type wick 200, a mesh-type wick 220 and a sintered-type wick 210.

It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. 

1. A heat pipe comprising: a metal casing comprising an evaporating section and a condensing section; a composite wick structure provided in the evaporating section, the composite wick structure comprising at least two wick layers in a radial direction of the heat pipe, one of the at least two wick layers being a plurality of fine grooves defined in an inner surface of the evaporating section; a singular wick structure provided in the condensing section, an average capillary pore size of the evaporating section being smaller than that of the condensing section.
 2. The heat pipe of claim 1, wherein the other one of the at least two wick layers is one of sintered powders and fine mesh.
 3. The heat pipe of claim 1, wherein the singular wick structure is a plurality of fine grooves.
 4. The heat pipe of claim 1, wherein the composite wick structure comprises three wick layers in the radial direction of the heat pipe from an outer wick layer of the heat pipe to an inner wick layer of the heat pipe, the three wick layers comprising a groove-type wick, a sintered-type wick and a mesh-type wick.
 5. The heat pipe of claim 1, wherein the composite wick structure comprises three wick layers in the radial direction of the heat pipe from an outer wick layer of the heat pipe to an inner wick layer of the heat pipe, the three wick layers comprising a groove-type wick, a mesh-type wick and a sintered-type wick.
 6. The heat pipe of claim 1, wherein the metal casing further comprises an adiabatic section provided between the evaporating section and the condensing section, the adiabatic section having a wick structure similar to the condensing section.
 7. A heat pipe comprising: a metal casing having an inner surface and defining an evaporating section for receiving heat and a condensing section for releasing heat; a working fluid received in the metal casing which can be evaporated into vapor in the evaporating section and condensed into liquid in the condensing section; and a composite capillary wick applied to the inner surface of the metal casing at the evaporating section and a singular capillary wick applied to the inner surface of the metal casing at the condensing section, the composite capillary wick comprising a groove-type wick, a mesh-type wick and a sintered-type wick, wherein the groove-type wick is firstly arranged in the inner surface of the evaporating section.
 8. The heat pipe of claim 7, wherein the groove-type wick has grooves defined in the inner surface of the evaporating section.
 9. The heat pipe of claim 8, wherein the mesh type wick is attached to the sintered-type wick at the evaporating section of the heat pipe.
 10. The heat pipe of claim 7, wherein the mesh type wick is attached to the groove-type wick of the evaporating section of the heat pipe.
 11. The heat pipe of claim 10, wherein the sintered-type wick is attached to the mesh type wick of the evaporating section of the heat pipe.
 12. A heat pipe comprising: a tubular metal casing having an evaporating section for receiving heat, a condensing section for releasing the heat and an adiabatic section between the evaporating section and the condensing section; a plurality of fine grooves functioning as a first type of wick structure being defined in an inner face of the metal casing through the evaporating, adiabatic and condensing sections; a second type of wick structure being attached to the fine grooves at the evaporating section; and a third type of wick structure being attached to the second type of wick structure at the evaporating section.
 13. The heat pipe of claim 12, wherein the second type of wick structure is a powder-sintered wick structure having a pore size larger than that of the fine grooves, and the third type of wick structure is a mesh-type wick structure having a pore size larger than that of the powder-sintered wick structure.
 14. The heat pipe of claim 12, wherein the second type of wick structure is a mesh-type wick structure having a pore size smaller than that of the fine grooves, and the third type of wick structure is a powder-sintered wick structure having a pore size smaller than that of the powder-sintered wick structure. 